The present invention relates to an electrifying apparatus which is used in an electrophotographic apparatus or an electrostatic recorder and imparts electric charge to toner used by an image carrier or for developing purposes, as well as to a photographic developing apparatus which visualizes an electrostatic latent image formed on the image carrier by applying toner to the image.
One-component developing equipment or two-component developing equipment is known as a photographic developing apparatus for visualizing a latent image formed on an image carrier by adhering toner to the latent image. Such developing equipment has a rotatable developing powder carrier positioned opposite the image carrier. A toner layer or a magnetic brush consisting of two-component developing powder is formed on the developing powder carrier and is carried to the developing area opposite the image carrier. An electric field is formed by applying a developing bias voltage between the developing powder carrier and the image carrier, thereby transferring toner to the latent image on the image carrier.
In order to properly apply the toner to the latent image on the image carrier in the developing area, the developing equipment must impart a predetermined amount of electric charges to the toner. In two-component developing equipment, electric charges are imparted to toner by mixing and stirring it with carriers which are separated from the toner in terms of friction and electrification ranks. A charge-control agent applied to the outside of toner particles may be liberated from the toner particles, and the thus-liberated charge-control agent may contaminate the surface of the carrier. For this reason, if the carrier is used for a long period of time, the liberation of the charge-control agent makes it impossible to impart an electric charge to the toner, whereby the developing powder must be exchanged. Further, a toner concentration controller or developing powder stirring equipment is needed to maintain a toner-carrier ratio constant. Further, a magnet must be provided in the developing powder carrier in order to hold a magnetic carrier, and this makes the developing equipment rather complicated.
In contrast, one-component developing equipment is widely used as an developing apparatus capable of overcoming the foregoing drawbacks of the two-component developing equipment. In one-component developing equipment, a toner layer forming member which is made of an elastic member and is called a blade is pressed against the developing powder carrier. A toner layer of one component is formed on the developing powder carrier, and an electric charge is imparted to the toner by friction and electrification from the toner layer forming member. The blade generally has a low electrification capability for toner, and hence it is difficult to sufficiently electrify all the toner particles. Therefore, there are so-called toner particles of the opposite polarity which are electrified with a polarity opposite to a desired polarity. These toner particles of the opposite polarity cause a background fog. Specifically, the frictional electrification caused by the blade provides a slight chance of bringing the toner in contact with the blade, and toner, particularly, fine toner, may be fed past the layer forming area without being frictionally electrified, so that the toner is not electrified. An increase in pressure for pressing the toner layer forming member against the toner is thought to promote frictional electrification. However, binder resin contained in the toner is fused by frictional heat, and the resultantly-condensed toner becomes attached to the blade, thereby resulting in white lines being formed on an image.
To solve the problems with developing equipment which frictionally electrifies the toner, there is proposed a developing apparatus which directly imparts an electric charge to the toner. Examples of such a developing apparatus are disclosed in; e.g., the Unexamined Japanese Utility Model Application Publication No. Sho 63-138560, the Japanese Patent Application Publication Nos. Sho 64-62675, Sho 54-17030, and Sho 62-291678, or illustrated in FIGS. 14 and 15.
The developing apparatus disclosed in the Unexamined Japanese Utility Model Application Publication No. Sho 63-138560 has a developing powder carrier and a corona discharge unit positioned so as to be opposite to it. An electric charge is imparted to the toner by exposing a toner layer to ions having a desired polarity. The use of the corona discharge unit results in soiling of a corotron wire, which in turn causes an uneven electric discharge. Further, a high voltage is necessary to ionize an air layer in the vicinity of the corotron wire. In terms of the stability of an electric discharge in the axial direction, a voltage of more than 5 kV is necessary. A large amount of ozone is produced at the time of an electric discharge.
In contrast, a developing apparatus illustrated in FIG. 14 is comprised of a developing powder carrier 202; a toner layer forming blade 203 which is pressed against the developing powder carrier to form a toner layer; a cylindrical electrifying member 204 which is supported while remaining in contact with or being spaced a very small distance away from the developing powder carrier 202; a toner supply member 205 which supplies toner to the developing powder carrier; a developing bias power source 206; and a toner electrifying power source 207. An electric discharge is produced across the gap between the cylindrical electrifying member 204 and the developing powder carrier 202 by applying a voltage to the cylindrical electrifying member 204 from the toner electrifying power source 207. Ions or electrons resulting from the electric discharge are adhered to the toner, whereby the toner is electrified.
The developing apparatus illustrated in FIG. 15 has a developing powder carrier 212 and a layer forming blade 213 positioned so as to be pressed against it. A voltage for electrifying toner is applied to the layer forming blade 213 from a power source 217, whereby an electric discharge is produced across the gap between the layer forming blade 213 and the developing powder carrier 212. A toner layer is formed on the developing powder carrier 212, and the toner is electrified.
The developing apparatus as illustrated in either FIG. 14 or 15 does not need the high voltage required for the corona discharge unit and produces only a small amount of ozone.
In a conventional electrophotographic apparatus or an electrostatic recording apparatus, a corona discharge device is commonly used for electrifying a photosensitive material or an image carrier. This corona discharge device is classified into corotrons and scorotrons. FIG. 48 is a schematic representation showing a scorotron type corona discharge device. This corona discharge device is principally made up of a discharge wire 102q laid so as to be opposite to an article to be electrified, and a conductive shield 103q provided so as to surround the discharge wire 102q. The discharge wire 102q has a line size of about several tens micrometers, and a high voltage is applied to the article from the discharge wire 102q. Ions are produced in the vicinity of the discharge wire 102q as a result of corona discharge, and the article is electrified by the migration of the thus-produced ions. Such a corona discharge device is capable of imparting a sufficient amount of discharged electric charges to the article by means of a simple structure.
Devices for electrifying a photosensitive material are disclosed in the Unexamined Japanese Patent Application Publication Nos. Sho 50-843, Sho 50-13661, Sho 64-73367, Sho 58-150975, Hei 4-51266, and Hei 4-249270.
In the devices disclosed in the Unexamined Japanese Patent Application Publication Nos. Sho 50-843, Sho 50-13661, and Sho 64-73367, a roller which is a resistive material is brought into contact with a photosensitive material, and electric discharge is continually produced in a minute gap by applying a voltage between the roller and the photosensitive material, whereby the photosensitive material is electrified. In the devices disclosed in the Unexamined Japanese Patent Application Publication Nos. Sho 58-150975, Hei 4-51266, and Hei 4-249270, a high-resistance conductive film is brought into contact with a photosensitive material, and electric discharge is produced in a minute gap by applying a voltage between the photosensitive material and the film.
Devices for electrifying toner are disclosed in the Unexamined Japanese Patent Application Publication Nos. Sho 60-83972, Sho 54-17030, Sho 62-29-1678, and Sho 64-62675.
In the device disclosed in the Unexamined Japanese Patent Application Publication No. Sho 60-83972, a plurality of electrodes are stacked on an insulating substrate in a side-by-side configuration, and they are positioned so as to be opposite to a developing roller having a thin layer of toner formed thereon. A discharge voltage is applied between the electrodes and the toner layer, thereby electrifying the toner provided on the developing roller.
In the devices disclosed in the Unexamined Japanese Patent Application Publication Nos. Sho 54-17030, Sho 62-291678, and Sho 64-62675, as shown in FIGS. 49A and 49B, a roller-shaped electrode 112q or a blade-shaped electrode 113q to which a voltage is applied is brought into contact with a toner layer formed on a developing roller 111q, and toner is electrified by electric discharge produced across a minute gap.
In general, in a photographic developing apparatus which employs one-component developing powder, toner is electrified by means of the friction between a developing roller and a toner layer regulating member pressed against the developing roller. It is difficult to impart sufficient electric charges to all toner particles by this method, and the toner of the opposite polarity is produced. To prevent the toner of the opposite polarity, there is proposed a technique of electrifying toner provided on the developing roller through use of the aforementioned electrifying device.
The toner of the opposite polarity is toner which is electrified with the polarity opposite to intended plurality. In a case where a latent image is developed by electrifying the toner with negative polarity, positively-electrified toner is the toner of the opposite polarity.
However, the developing equipment illustrated in either FIG. 14 or 15 has the following problem.
Specifically, if the voltage applied to the cylindrical electrifying member 204 is smaller than the discharge voltage, an electric charge is not sufficient because of the toner having high volume resistivity in the developing equipment illustrated in FIG. 14. Therefore, the toner cannot be electrified with the desired polarity. In contrast, toner can be electrified by means of electric discharge produced from electrodes by increasing the applied voltage. As shown in FIG. 16, electron avalanche occurs in the gap between the cylindrical electrifying member 204 and the developing powder carrier 202 as a result of ionization associated with electric discharge. Consequently, there are positive ions and electrons or negative ions which are opposite in polarity to each other in the discharge area. Similarly, electron avalanche occurs in the gap between the layer forming blade 213 and the developing powder carrier 212 in the developing equipment shown in FIG. 15, and there are eventually ions which are opposite in polarity to each other (see FIG. 17).
For these reasons, even if an attempt is made to adhere electrons or negative ions to toner in order to electrify the toner with a desired polarity (a negative polarity in this example), the discharge area is filled with positive ions and electrons or negative ions resulting from ionization. Therefore, both the positive ions and electrons or negative ions are imparted to the toner. As a result; the toner electrified as a result of electric discharge has a mixture of polarities; namely, particles of the opposite polarity are produced. The toner of the opposite polarity is thought to occur in the foregoing manner. The opposite polarity indicates a positive electric charge if the toner is desired to be electrified with a negative polarity as is in the present example. However, if the toner is electrified with a positive polarity, a negative polarity corresponds to the opposite polarity. A relative density between electrons and positive ions developing in the space between two parallel electrodes is calculated with reference to the paper entitled "Electric Discharge Phenomenon" (Tokyo Denki University Press., Atushi HONDA, pg. 64). As shown in FIG. 18, positive ions are also present in the vicinity of a positive electrode (i.e., the developing powder carrier), and the toner particle is several thousand times larger than an electron (i.e., the toner particle measures 7 to 10 .mu.m). In short, if an attempt is made to electrify toner with a negative polarity by electric discharge, a fair amount of positively charge toner is processed at the same time.
For example, as described in the article entitled "Recent Development and Actual Use of Electrophotographic Developing System and Toner" (Publishing Department of Nihon Kagaku Joho Co., Ltd., Manabu TAKEUCHI, pg. 303), the toner of the opposite polarity produced in the developing equipment is found to constitute 20% by weight by measuring the polarity of each of toner particles and the amount of electric charges in the form of profiles. If the toner particles of the opposite polarity are carried to the developing area as a result of the rotation of the developing powder carrier, picture degradation, such as background fog, arises in a developed toner image, resulting in a degraded image. Further, the toner of the opposite polarity on the developing powder carrier, splashes up and soils the inside of the developing equipment.
The present invention has been conceived in view of the foregoing problem, and the object of the present invention is to provide a photographic developing apparatus which produces an image with superior quality over a long period of time by preventing toner from being electrified with the opposite polarity and uniformly electrifying the toner on a developing powder carrier.
However, the previously-described photographic developing apparatus which directly imparts an electric charge to toner by applying a voltage to the toner present the following problems.
Specifically, if toner still remaining on a developing powder carrier is carried to the photographic developing apparatus after passage of a developing area, toner is additionally supplied to the residual toner, and an electric charge is again imparted to this toner by an electrifying device. As a result, an electric charge is imparted to the toner in small quantities as a result of electrification of the toner by the first rotation of a developing powder carrier, or in the first developing cycle. However, the amount of electric charges imparted to the toner shows a tendency to gradually increase in the next cycle and to become stable after passage of several cycles. The reason for this is that the toner still remaining on the developing powder carrier after completion of a developing operation is again carried to an electrifying area, thereby increasing the amount of electric charges of the toner. In contrast, toner newly applied to the developing powder carrier is not electrified in as large quantities as is the residual toner by one electrifying operation. Consequently, there arises a difference in the amount of electric charges among the toner particles.
As described above, the amount of electric charges imparted to the toner is instable. If the amount of electric charges changes from cycle to cycle, the quantity of toner to be transferred at the time of developing operation is also changed, thereby resulting in degradation of picture quality called ghosts.
Further, there is a case where an excessive electric current flows into the developing powder carrier as a result of local electric discharge developing between electrodes, toner is eventually electrified to such an extent that it would not contribute to the developing of an image. As a result, the quantity of toner to be transferred increases.
There is proposed a photographic developing apparatus as shown in, e.g., FIG. 28 or 29 as means for stabilizing the amount of electric charges imparted to developing powder.
As shown in FIG. 28, the photographic developing apparatus is comprised of a developing powder carrier 202p, a layer forming blade 203p which is pressed against the developing powder carrier 202p to thereby form a layer of toner; a cylindrical electric charge imparting member 204p which is supported so as to come into contact with the developing powder carrier 202p; a toner supply member 205p for feeding toner to the developing powder carrier 202p; a cylindrical conductive member 206p which is brought into pressed contact with the toner still remaining on the developing powder carrier after completion of the developing operation; a developing bias power source 207p; and a power source 208p for electrifying toner. The conductive member 206p is provided upstream from the toner supply member 205p in the direction of rotation of the developing powder carrier 202p and is electrically grounded.
In this photographic developing apparatus, electric discharge is caused across the gap between the electric charge imparting member 204p and the developing powder carrier 202p by applying a voltage to the electric charge imparting member 204p. Toner is electrified by applying ions or electrons caused by the electric discharge to the toner. An electrostatic latent image is developed through use of the toner, and the toner still remaining on the developing powder carrier after passage of the developing area is brought into contact with the conductive member 206p, thereby eliminating the electric charge of the toner. A photographic developing apparatus as described in, e.g., the Unexamined Japanese Patent Application Publication No. Hei 4-268587 is analogous to the foregoing photographic developing apparatus.
A photographic developing apparatus shown in FIG. 29 is comprised of a layer forming blade 213p which is positioned so as to be pressed against a developing powder carrier 212p; a power source 218p for applying a voltage used for electrifying toner; and a conductive film member 216p to be pressed against the toner provided on the developing powder carrier. The photographic developing apparatus causes electric discharge across the gap between the layer forming blade 213p and the developing powder carrier 212p, thereby forming a layer of toner on the developing powder carrier 212p and electrifying the toner. The conductive film member 216p eliminates the electric charge of the residual toner after passage of a developing area.
A charge removing mechanism of the photographic developing apparatus shown in FIG. 28 or 29 is merely a grounded conductive member. As a result of the conductive member physically coming into contact with the toner, a small degree of distortion occurs in the formation of the toner layer. However, the effect of reducing the extent to which the toner is electrified can be hardly expected.
The present invention has been conceived in view of the foregoing drawbacks, and the object of the present invention is to provide a photographic developing apparatus which ensures superior picture quality without ghosts by stabilizing the amount of electric charges imparted to toner provided on a developing powder carrier in each developing cycle.
However, the previously-described conventional electrifying devices have the following drawbacks.
Specifically, in the device which produces corona discharge, electric discharge develops in other directions in addition to the direction in which there is an electrode loaded with an insulating material to be electrified, and the device must be provided with a conductive shield in order to stabilize electric discharge. If this shield moves too close to a discharge electrode (i.e., the discharge wire) in an ionization area, spark discharge will be produced. For this reason, the shield must be spaced a given distance or more away from the discharge electrode. Since the size of the shield cannot be reduced so much, it is difficult to make the electrifying apparatus compact.
In an electrifying device in which a voltage is applied to a roller-shaped or film-shaped electrode consisting of a resistor, the electrode loaded with an article to be electrified is moved close to a resistor, and electric discharge is produced by means of an electric field directed toward the electrode. As a result, the article is imparted with an electric charge. Therefore, the gap between the resistor and the electrode must be set to a very small distance in order to produce a discharge electric field. In many cases, they are in contact with each other. Foreign articles adhered to the discharge electrode cause disturbances in the electrifying characteristics of the electrifying device. In short, in a case where this electrifying device is used for electrifying a photosensitive material, the electrifying characteristics of the electrifying device become changed or uneven by adhesion of a toner-including substance or paper dust still remaining on a photosensitive material to the discharge electrode or by abrasion of the surface of the discharge electrode.
In contrast, the following problems are encountered in electrifying toner through use of an electrifying device in which a voltage is applied to a roller-shaped or blade-shaped blade consisting of a resistor.
As shown in FIGS. 50A and 50B, a resistor electrode to which a voltage is applied is brought into contact with a toner layer, and electric discharge is produced between an electrode and a developing roller or between the electrode and a member for carrying toner. Therefore, the toner is present in the electric discharge occurring between them, so that the toner particles are not uniformly electrified with desired polarity; namely, so-called the toner of the opposite polarity is produced.
The following phenomenon will be considered as a mechanism to produce the toner of the opposite polarity.
Electron avalanche occurs as a result of ionization associated with electric discharge, whereby electric charges of opposite polarities such as positive ions and electrons or negative ions are produced. As illustrated in FIGS. 50A and 50B, since the toner is in the discharge area, both the positive ions and electrons or negative ions contribute to imparting of electric charges to the toner, thereby resulting in the toner of the opposite polarity.
For example, a relative density between electrons and positive ions produced in the space between two parallel electrodes is calculated with reference to the paper entitled "Electric Discharge Phenomenon" (Tokyo Denki University Press., Atushi HONDA, pg. 64). As shown in FIGS. 51A and 51B, positive ions are also present in the vicinity of a positive electrode (i.e., a developing powder carrier), and a toner particle is several thousand times larger than an electron (i.e., the toner particle measures 7 to 10 .mu.m). Further, as a result of the polarity of and the amount of electric particles of each toner particle being measured in the form of profiles by the method described in the article entitled "Recent Development and Actual Use of Electrophotographic Developing System and Toner" (Publishing Department of Nihon Kagaku Joho Co., Ltd., Manabu TAKEUCHI, pg. 303), the toner of the opposite polarity constitutes 20% by weight.
The electrifying device disclosed in the Unexamined Japanese Patent Application No. Sho 60-83972 also has similar problems.
The present invention has been conceived in view of the foregoing drawbacks, and the object of the invention is to provide an electrifying apparatus which prevents foreign articles from sticking to the electrifying apparatus by keeping the electrifying apparatus from contact with an article to be electrified, and which can be made compact.
Another object of the present invention is to provide a compact electrifying apparatus which prevents the generation of the toner of the opposite polarity by imparting an electric charge of single polarity to toner when it is used for electrifying the toner, and which can form an image with a stable picture quality over a long period of time in an image forming apparatus.
"The present invention has been conceived in view of the foregoing drawbacks in the prior art, and the object of the present invention is to provide a photographic developing apparatus which substantially uniformly electrifies toner provided on a developing powder carrier by preventing generation of toner of the opposite polarity, and which ensures superior picture quality over a long period of time. Another object of the present invention is to provide a photographic developing apparatus which ensures superior picture quality without ghosts by stabilizing the amount of electric charge of toner provided on a developing powder carrier every developing cycle. Further object of the present invention is to provide an electrifying apparatus which prevents attachment of foreign articles by keeping it out of contact with an article to be electrified and can be rendered compact.